A commonly used method for forming metal lines and vias is known as “damascene.” Generally, this method involves forming an opening in a dielectric layer, which separates the vertically spaced metallization layers. The opening is typically formed using conventional lithographic and etching techniques. After the formation, the opening is filled with copper or copper alloys to form a via or a trench. Excess metal material on the surface of the dielectric layer is then removed by chemical mechanical polish (CMP). The remaining copper or copper alloy forms vias and/or metal lines.
Copper is typically used in the damascene process because of its lower resistivity. However, copper suffers from electro-migration (EM) and stress-migration (SM) reliability issues, particularly as geometries continue to shrink and current densities continue to increase. Therefore, barrier layers are typically formed to prevent copper from diffusing into neighboring low-k dielectric materials. Recently, copper silicide nitride layers are increasingly used as barrier layers.
FIG. 1 illustrates a cross-sectional view of an intermediate stage in the formation of a conventional interconnect structure. Copper line 4 is formed in a low-k dielectric layer 2. Copper silicide layer 6, which acts as a barrier layer, is formed on the top surface of copper line 4 by exposing copper line 4 to silane plasma. Subsequently, copper silicide layer 6 is nitridated to form a copper silicide nitride layer by treating the structure shown in FIG. 1 in nitrogen-containing plasma (such as NH3 plasma).
The conventional formation process of barrier layers suffers drawbacks. Since copper silicide is relatively unstable, silicon may still break from copper silicide and diffuse into low-k dielectric layer 2. Therefore, it is preferred that copper silicide layer 6 is fully nitridated to form copper silicide nitride, which is more stable. This requires long NH3 plasma treatment and/or high power. However, plasma treatments have the side effect of incurring damage to low-k dielectric layer 2. FIG. 2 schematically illustrates damaged low-k dielectric layers 8, which are damaged portions of low-k dielectric layer 2 adjacent copper line 4. Typically, due to plasma treatment, carbon is depleted from low-k dielectric layers 8, and thus the k value of dielectric layers 8 increases. To reduce the damage to low-k dielectric layer 2, shorter plasma treatment and/or lower power are preferred.
The conflicting requirements to the plasma treatment time and power leave a small process window for nitrogen-containing plasma treatment. It is difficult to control the formation of copper silicide nitride layer without incurring the side effects. Therefore, what is needed in the art is an interconnect structure and formation methods that may incorporate barrier layers thereof to take advantage of the benefits associated with the reduced copper diffusion while at the same time overcoming the deficiencies of the prior art.